The maintenance of genomic integrity relies on the efficacy of DNA repair systems. These systems counteract the mutational burden imposed on DNA by exogenous attacks, endogenous reactive species, and errors originating during replication. Failure of DNA surveillance and repair mechanisms leads to an increase in the mutation rate, and this, in turn, results in predisposition to cancer. A prominent role in mutational avoidance and genomic stability is performed by the DNA mismatch repair system. This system handles base pair mismatches, short insertions/deletions and recombination-derived heteroduplexes. Patients with Hereditary Non-Polyposis Colorectal Cancer (HNPCC) carry a germline mutation in genes involved in DNA mismatch repair (h MSH2, h MLH1, GTBP /hMSH6, hPMS2 and hPMS1). These genes encode human homologues of the E. coli mismatch repair proteins MutS and MutL. In the bacterial system, a third protein, the single-strand endonuclease MutH, performs the crucial function of strand recognition, incising the newly synthesized DNA strand carrying the mutation. The new strand is identified by virtue of the transient lack of adenine methylation at GATC sites. To date, eukaryotic homologues of MutH, i.e. eukaryotic mismatch repair endonucleases, have not been identified, and the molecular determinants of strand discrimination in eukaryotic cells - which lack GATC methylation - have remained elusive. By employing the yeast interaction trap with hMLH1 as bait , MED1 (mismatch repair endonuclease1), a novel human gene encoding a protein with homology to bacterial endonucleases, was cloned. Sequence analysis of MED1 suggests a possible mechanism of strand recognition based on cytosine methylation at CpG sites. For its interaction with hMLH1 and homology to bacterial DNA repair proteins, MED1 is a putative mismatch repair protein and might be a long sought eukaryotic functional homologue of MutH. Since mismatch repair genes are mutated in HNPCC and sporadic cancers with microsatellite instability, MED1 is a candidate gene for cancer genetic testing. Based in these observations, experiments are proposed to address: 1) the biochemical properties of MED1; 2) its functional role in DNA repair. These studies may provide new insights into the mechanisms of eukaryotic mismatch repair and further the link between defective DNA repair and cancer.